A thermal flow meter that measure a flow rate of gas is configured to include an air flow sensing portion for measuring a flow rate, such that a flow rate of the gas is measured by performing heat transfer between the air flow sensing portion and the gas as a measurement target. The flow rate measured by the thermal flow meter is widely used as an important control parameter for various devices. The thermal flow meter is characterized in that a flow rate of gas such as a mass flow rate can be measured with relatively high accuracy, compared to other types of flow meters.
However, it is desirable to further improve the measurement accuracy of the gas flow rate. For example, in a vehicle where an internal combustion engine is mounted, demands for fuel saving or exhaust gas purification are high. In order to satisfy such demands, it is desirable to measure the intake air amount which is a main parameter of the internal combustion engine with high accuracy. The thermal flow meter that measures the intake air amount guided to the internal combustion engine has a bypass passage that takes a part of the intake air amount and an air flow sensing portion arranged in the bypass passage. The air flow sensing portion measures a state of the measurement target gas flowing through the bypass passage by performing heat transfer with the measurement target gas and outputs an electric signal representing the intake air amount guided to the internal combustion engine. This technique is discussed, for example, in JP 2011-252796 A (PTL 1).
Here, an element (a resistor) and the like are arranged to detect the flow rate in an air flow sensing area of an air flow sensing element corresponding to the air flow sensing portion of the thermal flow meter, and a gap is formed in the rear surface of the air flow sensing element corresponding to the air flow sensing area. With this configuration, since a thin diaphragm is formed in a portion including the air flow sensing area, a heat capacity of the air flow sensing element is reduced, and a response of the thermal flow meter is improved. For example, such a technology is disclosed in WO 2002-101836 A1 (PTL 2).